How to set end time based on the third value?

How to set end time based on the third value?

Hello everyone, I'm trying to get my end time drop down values based on the length of meeting that I can pick. So my code look like this:
<tr>
       <th>Start:</th>
       <td><input type="text" id="start" name="start"/></td>
</tr>
<tr>
      <th>Length of meeting</th>
      <td>
<select name="meeting" id="meeting" onClick="interval()">
      <option value="">--Select length--</option>
</select>
      </td>
</tr>
<tr>
<th>End:</th>
<td><input type="text" id="end" name="end"></td>
</tr>

<script>
$(function() { 
$('#start').timepicker({
'minTime':'8:00 AM',
'maxTime':'9:00 PM'
});

$('#end').timepicker({
'minTime':'8:00 AM',
'maxTime':'9:00 PM',
'step':???(how to pass my value from meeting dropdown here?)
});
});
</script>

So I can pick start time, then I can pick the length of my meeting that is created dynamically and then I should get my end time. Intervals of my meetings are from 5 to 60 min. So if I pick my start time 8:00am and my meeting length 15min, I should get my end time in intervals of 15min. So my first value in end dropdown will be 8:15am, then 8:30am  and so on. For now I was able to hard code mt value in end timepicker 'step':15. What I need is that I want to pass my meeting length value and that way increment my end time. If anyone can help with this I would appreciate. Thanks in advance.